It would increase the hilarity of losing 1 to a vengeance spell
Otherwise, it doesn't seem to be very consequential to me either way. The only thing i can think of is bluffing the "Oh shite! I don't have enough mana to cast this mega spell! I just need a bit more time to burn cards so that i for sure have a real cast!"

but the same could be said if a megaspell just popped out without the indicator.

I guess it just acts as more of a means to add a theatrical effect. I can't remember ever activating a mega spell and not casting it.
Although..........if there was a spell that allowed players to suck up another player's mana...that would be a whole other story

There was a period when the mega-spell activation was kept secret, in retrospect I think it might be better the way it is now. For continuity you want the log to display all types of activations.

If you do activate it early (many turns before intending to cast it) you are effectively announcing the activation (you could use this as a way of sending out a warning)..

..Rarely do players actually activate it on the turn before they intend to cast it though (unless they are playing the AI), generally players will activate it on the turn they intend to cast it, the risk of losing the pre-activated mega-spell card to an MA is risky. Plus if you activate it but don't cast it you wont draw a new spell card that turn (if you have spells to come).

So adding the activation to the log did introduce some different tactics with regard to when you choose to activate it.